On Fri, 11 Feb 2022, Patrick Palka wrote:
> In the P2325R3 backport r11-9555 the relaxation of the constraints on
> the partial specialization of __box (which is semantically equivalent to
> the primary template, only more space efficient) means some
> specializations of __box will now use the partial specialization instead
> of the primary template, which (IIUC) constitutes an ABI change unsuitable
> for a release branch. This patch reverts this constraint change, which
> isn't needed for correctness anyway.
>
> Similarly the change to use __non_propagating_cache for the data member
> split_view::_M_current (so that it's always default-initializable) also
> constitutes an unsuitable ABI change. This patch reverts this change
> too, and instead further constrains split_view's default constructor to
> require that we can default-initialize _M_current.
Forgot to clarify that this is for the 11 branch, tested on
x86_64-pc-linux-gnu. Does this look reasonable? I noticed these
issues while backporting r11-9555 to the 10 branch, which doesn't have
__non_propagating_cache or the partial specialization of __box.
>
> PR libstdc++/103904
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges (__detail::__box): Revert r11-9555 changes
> to the constraints on the partial specialization and the
> now-unnecessary member additions.
> (__detail::__non_propagating_cache::operator=): Remove
> now-unused overload added by r11-9555.
> (split_view::_OuterIter::__current): Adjust after reverting the
> r11-9555 change to the type of _M_current.
> (split_view::_M_current): Revert r11-9555 change to its type.
> (split_view::split_view): Constrain the default ctor further.
> * testsuite/std/ranges/adaptors/detail/copyable_box.cc: Disable
> now-irrelevant test for the r11-9555 changes to the partial
> specialization of __box.
> ---
> libstdc++-v3/include/std/ranges | 54 +++----------------
> .../ranges/adaptors/detail/copyable_box.cc | 4 ++
> 2 files changed, 10 insertions(+), 48 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 03c6275801f..bf31e4be500 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -144,8 +144,7 @@ namespace ranges
> // std::optional. It provides just the subset of the primary template's
> // API that we currently use.
> template<__boxable _Tp>
> - requires copyable<_Tp> || (is_nothrow_move_constructible_v<_Tp>
> - && is_nothrow_copy_constructible_v<_Tp>)
> + requires copyable<_Tp>
> struct __box<_Tp>
> {
> private:
> @@ -174,38 +173,6 @@ namespace ranges
> : _M_value(std::forward<_Args>(__args)...)
> { }
>
> - __box(const __box&) = default;
> - __box(__box&&) = default;
> - __box& operator=(const __box&) requires copyable<_Tp> = default;
> - __box& operator=(__box&&) requires copyable<_Tp> = default;
> -
> - // When _Tp is nothrow_copy_constructible but not copy_assignable,
> - // copy assignment is implemented via destroy-then-copy-construct.
> - constexpr __box&
> - operator=(const __box& __that) noexcept
> - {
> - static_assert(is_nothrow_copy_constructible_v<_Tp>);
> - if (this != std::__addressof(__that))
> - {
> - _M_value.~_Tp();
> - std::construct_at(std::__addressof(_M_value), *__that);
> - }
> - return *this;
> - }
> -
> - // Likewise for move assignment.
> - constexpr __box&
> - operator=(__box&& __that) noexcept
> - {
> - static_assert(is_nothrow_move_constructible_v<_Tp>);
> - if (this != std::__addressof(__that))
> - {
> - _M_value.~_Tp();
> - std::construct_at(std::__addressof(_M_value), std::move(*__that));
> - }
> - return *this;
> - }
> -
> constexpr bool
> has_value() const noexcept
> { return true; };
> @@ -1180,16 +1147,6 @@ namespace views::__adaptor
> return *this;
> }
>
> - constexpr __non_propagating_cache&
> - operator=(_Tp __val)
> - {
> - this->_M_reset();
> - std::construct_at(std::__addressof(this->_M_payload._M_payload),
> - std::in_place, std::move(__val));
> - this->_M_payload._M_engaged = true;
> - return *this;
> - }
> -
> constexpr _Tp&
> operator*() noexcept
> { return this->_M_get(); }
> @@ -2886,7 +2843,7 @@ namespace views::__adaptor
> if constexpr (forward_range<_Vp>)
> return _M_current;
> else
> - return *_M_parent->_M_current;
> + return _M_parent->_M_current;
> }
>
> constexpr auto&
> @@ -2895,7 +2852,7 @@ namespace views::__adaptor
> if constexpr (forward_range<_Vp>)
> return _M_current;
> else
> - return *_M_parent->_M_current;
> + return _M_parent->_M_current;
> }
>
> _Parent* _M_parent = nullptr;
> @@ -3143,13 +3100,14 @@ namespace views::__adaptor
> // XXX: _M_current is "present only if !forward_range<V>"
> [[no_unique_address]]
> __detail::__maybe_present_t<!forward_range<_Vp>,
> - __detail::__non_propagating_cache<iterator_t<_Vp>>> _M_current;
> + iterator_t<_Vp>> _M_current;
> _Vp _M_base = _Vp();
>
>
> public:
> split_view() requires (default_initializable<_Vp>
> - && default_initializable<_Pattern>)
> + && default_initializable<_Pattern>
> + && default_initializable<iterator_t<_Vp>>)
> = default;
>
> constexpr
> diff --git
> a/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc
> b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc
> index fa6d4d56816..2078d442447 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/copyable_box.cc
> @@ -101,6 +101,9 @@ test02()
> __box<A<false>> x1(std::in_place, 0, 0);
> }
>
> +#if 0
> +// On the 11 branch, the partial specialization of __box admits only
> copyable types
> +// so this test doesn't apply.
> constexpr bool
> test03()
> {
> @@ -142,3 +145,4 @@ test03()
> return true;
> }
> static_assert(test03());
> +#endif
> --
> 2.35.1.102.g2b9c120970
>
>
